Recovery of precious metals and copper from copper/gold ores using resin technology

ABSTRACT

A process for the selective recovery of precious metals from a medium loaded with precious metals and base metals. The selective recovery of precious metals is achieved by exposing the medium to an aqueous solution containing a complex of formula MX 2 —, wherein M is a metal of group IB or IIB of the Periodic Table of Elements, and X is a monovalent radical. A second invention relates to the removal of copper from the medium whereby elution of copper is achieved by exposing the medium to an aqueous solution capable of oxidising Cu(I) to Cu(II) whilst not eluting precious metals from the medium.

TECHNICAL FIELD

[0001] The present invention relates to a hydrometallurgical process for selectively recovering precious metals from a medium loaded with precious metals and base metals. Further, the present invention also relates to a process for eluting copper from a medium loaded with precious metals, copper and optionally, other base metals.

BACKGROUND ART

[0002] The Australian gold industry currently produces more than 320 tonnes of gold a year. However, as the resources of the high grade “oxidised” gold ores are dwindling, the industry is facing more difficult tasks ahead in the future. One of the problems currently faced by mining operations is related to the processing of gold-bearing ores containing cyanide-soluble copper minerals.

[0003] A wide range of gold-bearing ores has been found in Australia, which are not economical to process due to the interference of dissolved copper cyanide species. The dissolution of cyanide-soluble copper minerals from copper-gold ores, results in high cyanide consumption and subsequently high levels of copper in the leach solutions. Apart from the higher operating costs incurred, the recovery of gold from a copper-rich cyanide liquor presents serious technical difficulties as copper cyanide is co-adsorbed onto the activated carbon or resin used during the adsorption stages. As copper is extracted into the cyanide leach reaching several orders of magnitude higher in concentration compared to gold (500 mg/L Cu(I) compared to 5 mg/L Au(I)), the adsorbent used for gold recovery from solution is “swamped” with copper. This copper is then co-eluted with gold in the next stage, rendering gold solutions containing a significant amount of copper, hindering the final stage of gold recovery by electrolysis or zinc cementation.

[0004] An approach to recover gold from copper-gold cyanide mixtures involves the use of adsorbent resin technology. Previous desorption (elution) methods for removing precious metal cyanoions from strong base anion exchange resins have relied either on the reversibility of the adsorption exchange reaction or the chemical conversion of the adsorbed metal anion to a cationic species, as derived for example from acid thiourea eluants.

[0005] Typically the reversal of the adsorption process requires the use of an aqueous eluant containing high concentrations of simple anions such as cyanide, chloride, bisulphate, sulphate, nitrate or thiocyanate, wherein the strong affinity of the resin for the precious metal anions is overcome by the mass action impact of the eluant anions. The use of the zinc tetracyanide divalent anion in aqueous solutions to elute adsorbed gold and silver cyanoanions is in a similar category, with the added attraction being the strong affinity for that anion exhibited by strong base anion resins. This use invariably involves heated aqueous solutions of sodium zinc tetracyanide operating in a closed electroelution circuit where the eluate from the resin bed enters the electrowinning cell as the electrolyte, and emerges diminished in precious metal content to be recycled back to the resin bed once more as the eluant. The precious metal elution efficiency is directly related to the operating efficiency of the electrowinning circuit wherein the return precious metal concentrations have to be reduced to very low values to increase the driving force of the elution process. Typically a period of between two to three days is required for the elution efficiency to reach 95%.

[0006] Following the elution of the precious metals in this manner, the resin is saturated with adsorbed zinc tetracyanide to the extent that it has replaced all removed anions, for example 90 to 100 kg/t of dry resins. Considering that the precious metal loading on the resin in the first place might have amounted to only 5 to 10 kg/t this represents a practical and economic problem because the adsorbed zinc tetracyanide has to be removed, generally by washing in a sulphuric acid solution. The product is gaseous HCN and an acidic zinc sulphate solution which also contains HCN, and both products represent major economic and environmental problems.

[0007] The present invention provides an improved precious metal elution process. At least some of the advantages of the present invention arise through the penetration of complex eluting metal cyanions from the external liquid medium to replace the adsorbed precious metal anions resident within the charged liquid film at the resin surface. This penetration being greatly enhanced due to the eluting anions possessing a linear structure, thereby favouring faster exchange kinetics than that achieved with the bulkier and more slowly diffusing tetrahedral zinc tetracyanide anion. Also, the presence of a heavy transition metal ion within the complex eluting cyanions contributes to the attractive force developed between the fixed and mobile ions, in addition to the electrostatic forces between the oppositely charged entities at the resin surface.

[0008] The present invention adapts this concept to solving the problem of desorbing (eluting) adsorbed precious metal cyanions, and overcomes the following deficiencies in current practice:

[0009] (i) since a cyanided gold/copper liquor commonly exhibits a base to precious metal ratio of between 50 to 1000 to one, the existing processes do not allow copper to be selectively and fully eluted from the resin, even prior to the eventual elution of the precious metals.

[0010] (ii) current technologies rely on strong alkaline cyanide solutions constituted from sodium cyanide or zinc tetracyanide to elute copper as cyanide from the resins. These elution systems present great difficulty and complexity in operation over the many adsorption and elution cycles commonly used in practice. Moreover, the high CN concentrations in the eluates lead to difficulties in controlling the regenerated HCN gas emissions. Further, the by-product, CuCN, typically produced by such processes is not an accepted commercial product. Also, of particular concern with these high strength cyanide eluants is their tendency to remove unacceptably high concentrations of precious metals along with the copper.

[0011] (iii) in the past any gold and other precious metal cyanides have to be eventually eluted from the resins for recovery, using zinc tetracyanide, thiocyanate or thiourea eluant. Subsequent gold recovery whether by electro or hydrometallurgy is complicated by these reagents and is basically inefficient.

[0012] (iv) a regeneration stage is required for each resin charge following elution operations, whether it be for base or precious metals, to wash off all residual adsorbed cyanides (copper, zinc, free cyanide or thiocyanate), before the resin is re-used in the next adsorption cycle. Great care has to be take during this stage if the resins are not to be adversely affected by the deposition of insoluble products within their structure which lead to a severe decrease in resin adsorption efficiency after each cycle.

[0013] Accordingly, there is a need for a process for the selective recovery of precious metals from a medium loaded with precious and base metals, which does not involve the use of high concentrations of cyanide, and which provides faster exchange kinetics than that achieved with the bulkier and more slowly diffusing tetrahedral zinc tetracyanide anion. Further, there is a need for a process for the elution of copper from the medium, either before or after precious metal recovery.

DISCLOSURE OF THE INVENTION

[0014] According to a first embodiment of the invention, there is provided a process for the selective recovery of precious metals from a medium loaded with precious metals and base metals, wherein said selective recovery is achieved by exposing said medium to an aqueous solution containing a complex of formula MX₂—, wherein M is a metal of group IB or IIB of the Periodic Table of Elements, and X is a monovalent radical.

[0015] According to a second embodiment of the invention, there is provided a process for the elution of copper from a medium loaded with precious metals, copper and optionally other base metals, wherein said elution is achieved by exposing said medium to an aqueous solution capable of oxidising Cu(I) to Cu(II), whilst not eluting said precious metals from said medium.

[0016] According to a third embodiment of the invention, there is provided a process for the elution of copper from a medium loaded with precious metals, copper and optionally other base metals, wherein said process comprises:

[0017] (a) selectively recovering said precious metals from said medium in accordance with the first embodiment of the invention, and

[0018] (b) eluting said copper from said medium by exposing said medium to an aqueous solution capable of oxidising Cu(I) to Cu(II).

[0019] According to a fourth embodiment of the invention, there is provided a process for the selective recovery of precious metals from a medium loaded with precious metals and base metals, wherein said process comprises:

[0020] (a) eluting copper from said medium in accordance with the second embodiment of the invention, and subsequently

[0021] (b) selectively recovering precious metals by exposing said medium to an aqueous solution containing a complex of formula MX₂— in accordance with the first embodiment of the invention.

[0022] The following features relate to any one of the first, second, third or fourth embodiments of the invention.

[0023] Typically, the precious metal is gold, silver or platinum group metals.

[0024] Typically, the precious metals are present on the medium in the form of precious metal cyanoion complexes.

[0025] Typically, the precious metals are removed from the medium in the form of precious metal cyanoion complexes.

[0026] Typically, copper and other base metals are present on the medium in the form of copper and other base metal cyanoion complexes. Typically. the copper and other base metals are removed from the medium in the form of copper and other base cyanoion complexes.

[0027] Typically, the base metals are selected from the group consisting of: copper, nickel, cobalt, zinc, iron and lead.

[0028] Typically, the medium is an ion exchange medium or activated carbon. The ion-exchange medium may be any commercially available ion exchange medium. More typically, the ion exchange medium may be of a salt, acid or base in nature, and is generally solid in form. Even more typically, the ion exchange medium is an anion exchange medium. Still more typically, the anion exchange medium is in the form of an anion exchange resin.

[0029] Still even more typically, the anion exchanger functional group of an ion exchange resin for use in the process of the present invention is a strongly basic benzyltrimethylammonium chloride.

[0030] The following features relate to the first or fourth embodiments of the invention.

[0031] Typically, in the complex anion of formula MX₂-, the metal M is selected from the group consisting of: copper(I), gold(I), silver(I), cadmium(I) and mercury(II). More typically, in the complex anion of formula MX₂—, the metal M is copper.

[0032] Typically, X corresponds to a monovalent anion selected from the group consisting of: halides, cyanide, thiocyanate and isothiocyanate. The halide may be selected from the group consisting of: chloride, bromide, iodide and fluoride. Even more typically, X is selected from cyanide and thiocyanate.

[0033] Still more typically, the complex anion of formula MX₂ _(^(x)) is either Cu(CNS)₂— or Cu(CN)₂—. Still even more typically, the complex anion of formula MX₂— is Cu(CN)₂—.

[0034] Typically, the complex anion of formula MX₂ ^(—) is linear.

[0035] It will be appreciated that the aqueous solution of the complex anion of formula MX²— also includes a counterion. The counterion may be any counterion which does not cause precipitation of the complex anion of formula MX₂—. For example, such a counterion may include an alkali metal cation, for instance, Na⁺ or K⁺, or an ammonium ion.

[0036] Typically, the aqueous solution containing a complex anion of formula MX₂— also contains a salt of formula M′X, wherein said M′ may be, but is not necessarily, the counter cation in relation to the complex anion of formula MX₂—.

[0037] Generally, the amount of the salt of formula M′X is from about 0 to about 10 moles per mole of the complex anion of formula MX₂—. More typically, the amount of the salt of formula M′X per mole of the complex anion of formula MX₂′ is in the range of between any one of the following: between about 0.1 to about 8 moles per mole, about 0.1 to about 6 moles per mole, about 0.1 to about 5 moles per mole, about 0.1 to about 4 moles per mole, about 0.1 to about 3 moles per mole, about 0.1 to about 2.5 moles per mole, about 0.1 to about 2 moles per mole, about 0.1 to about 1.5 moles per mole, about 0.1 to about 1 moles per mole, about 0.2 to about 5 moles per mole, about 0.2 to about 4 moles per mole, about 0.2 to about 3 moles per mole, about 0.2 to about 2.5 moles per mole, about 0.2 to about 2 moles per mole, about 0.5 to about 1.5 moles per mole, about 0.2 to about 1 moles per mole. Still more typically, the amount of the salt of formula M′X is about 1 mole per mole of the complex anion of formula MX₂—.

[0038] Typically, the minimum amount of aqueous solution containing a complex anion of formula MX₂— used in the present invention is that sufficient to elute the precious metals from the ion exchange medium or activated carbon medium. Generally, the lower limit of the concentration of the complex anion of formula MX₂— in the aqueous solution depends on the amount of precious metals to be eluted off the ion exchange medium or activated carbon medium, and the desired rate of elution.

[0039] Typically, the upper limit of the concentration of the complex anion of formula MX₂— in the aqueous solution is that wherein the complex anion of formula MX₂— remains soluble. Generally, the concentration of the complex anion of formula MX₂— in the aqueous solution wherein the complex anion of formula MX₂— remains soluble is dependent upon a number of factors, including temperature and the X/M molar ratio. More typically, the concentration of the complex anion of formula MX₂— in the aqueous solution is at least about 0.01M. Even more typically, the concentration of the complex anion of formula MX₂— in the aqueous solution is in the range of between any one of the following: about 0.01M to about 2M, about 0.01M to about 1.75M, about 0.01M to about 1.5M, about 0.01M to about 1.25M, about 0.01M to about IM, about 0.01M to about 0.75M, about 0.01M to about 0.5M, about 0.05M to about 2M, about 0.05M to about 1.75M, about 0.05M to about 1.5M, about 0.05M to about 1.25M, about 0.05M to about 1M, about 0.05M to about 0.75M, about 0.05M to about 0.5M, about 0.1M to about 2M, about 0.1M to about 1.75M, between about 0.1M to about 1.5M, about 0.1M to about 1.25M, about 0.1M to about IM, about 0.1M to about 0.75M, and yet even still more typically, the concentration of the complex anion of formula MX₂— in the aqueous solution is in the range of between about 0.1M to about 0.5M.

[0040] Usually, the aqueous solution of the complex anion of formula MX₂— is provided by preparing an aqueous solution of a salt M′X at the desired concentration, wherein M′ is the desired counterion, and adding a salt MX in a sufficient quantity to provide the desired concentration of M, and hence the complex anion of formula MX₂—, in solution.

[0041] Alternatively, an aqueous solution of MX may be prepared first, with subsequent addition of M′X, or both MX and M′X may be added to water together, or any combination of these steps may be used.

[0042] Other reaction conditions for carrying out the process of the present invention are not critical. The process may be carried out at ambient temperature, or elevated temperatures, but is typically carried out at ambient temperature. The pH is usually the pH of the aqueous solution of the complex anion of formula MX₂— as prepared, without any further pH adjustment.

[0043] The processes of the present invention are useful for mediums loaded with precious metal-cyano complexes via both clear and/or slurried solutions.

[0044] The following features relate to the second or third embodiments of the invention.

[0045] Once the precious metals have been eluted off the medium, they may then be cemented onto copper metal by methods generally known in the art. Precious metal/copper cement can then be easily purified by dissolution of copper in a mixture of sulphuric acid and an oxidant (such as hydrogen peroxide or air), leaving behind a product rich in precious metals for smelting into doré. Further, methods for dissolution of copper from cement containing precious metals are also generally known in the art.

[0046] The process engineering conditions chosen for contacting the resin and solution will be a design variable but preferably will incorporate either a column resin packed bed or some suitable form of stirred liquid/solids reactor configured for either batch or continuous operation. Typically, solution contacts the resin via either a series of elution cycles, or a continuous passage of recycled solution.

[0047] Typically, any base metal-cyano complexes are removed from the medium together with the copper metal-cyano complexes. More typically, the base metals are selected from the group consisting of: copper, nickel, cobalt, zinc, iron and lead.

[0048] Typically, in relation to the second or third embodiment of the invention, the aqueous solution capable of oxidising Cu(I) to Cu(II) comprises an oxidising agent, together with an acid. More typically, the oxidising agent is selected from the group consisting of: hydrogen peroxide, Caro's acid (H₂SO₅) and acid ferric sulphate, and the acid is a mineral acid. Even more typically, the mineral acid is selected from the group consisting of: sulphuric acid, hydrochloric acid and nitric acid.

[0049] Typically, the minimum amount of aqueous solution of oxidising agent used in the present invention is that sufficient to oxidise Cu(I) to Cu(II). More typically, the aqueous solution capable of oxidising Cu(I) to Cu(II) comprises between about 0.25-5% by weight hydrogen peroxide and about 1-20% by weight sulphuric acid; more typically between about 0.254% by weight hydrogen peroxide and about 1-15% by weight sulphuric acid; even more typically between about 0.25-3% by weight hydrogen peroxide and about 1-12% by weight sulphuric acid; still more typically between about 0.25-2.5% by weight hydrogen peroxide and about 1-10% by weight sulphuric acid; yet still more typically between about 0.25-2% by weight hydrogen peroxide and about 1-8% by weight sulphuric acid; even still more typically between about 0.35-2% by weight hydrogen peroxide and about 2-8% by weight sulphuric acid; yet still more typically between about 0.35-1.5% by weight hydrogen peroxide and about 2-7.5% by weight sulphuric acid; even more typically between about 0.25-1% by weight hydrogen peroxide and about 2.5-7.5% by weight sulphuric acid; even more typically between about 0.35-1% by weight hydrogen peroxide and about 2.5-6.5% by weight sulphuric acid; even more typically between about 0.4-0.8% by weight hydrogen peroxide and about 3-6% by weight sulphuric acid; yet still more typically, the aqueous solution capable of oxidising Cu(I) to Cu(II) comprises about 0.5% by weight hydrogen peroxide and about 5% by weight sulphuric acid.

[0050] Typically, the aqueous eluant contacts the loaded resin at ambient temperatures for periods up to about 8 hours, depending on the desired extent of copper elution. More typically, the contact time is between about 1 and 5 hours, even more typically between about 1 and 3.5 hours, still more typically, about 2 hours.

[0051] Typically, prior to exposing the medium to the aqueous solution capable of oxidising Cu(I) to Cu(II), the medium is contacted with a mineral acid. More typically, the mineral acid is selected from the group consisting of: sulphuric acid, hydrochloric acid and nitric acid. Even more typically, the mineral acid is sulphuric acid. Typically, the mineral acid may be present at between 1 to 10% by weight, more typically, between about 1 to about 7.5% by weight; even more typically, between about 1 to about 5% by weight; still more typically, between about 1 to about 4% by weight; and still more typically, between about 1 to about 2.5% by weight.

[0052] Typically, cyanide liberated as HCN from the adsorbed cyano complex after washing the medium, typically an anion exchange medium, in accordance with the second or third embodiments of the invention, is removed from the system, together with the acid solution, by passing the HCN gas directly into a solution of calcium or sodium hydroxide. More typically, the cyanide liberated as HCN from the adsorbed cyano complex is removed from the system by passing into an air purged volatilisation column wherein the HCN gas is removed into a caustic soda scrubbing tower for the regeneration of sodium cyanide.

[0053] Typically, the recovery of copper metal from resultant acid solution after exposing the medium in accordance with either the second or third embodiments of the invention, may be facilitated using any commercially available technology in this field. More typically, recovery of copper metal from the resultant acid solution after exposing the medium may be obtained by electrowinning to cathodic copper. Even more typically, the recovery of copper metal from the resultant acid solution after exposing the medium may be obtained by cementation on materials such as scrap ferrous metal, or recovered as copper hydroxide by precipitation in alkaline solution.

[0054] Typically, the medium is not damaged or destroyed by repeated contact with the aqueous solution capable of oxidising Cu(I) to Cu(II).

[0055] In the context of this specification, the term “comprising” means “including principally, but not necessarily solely”. Further, variations of the word “comprising”, such as “comprise” and “comprises” have correspondingly varied meanings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 illustrates gold elution from anion exchange resins using CuCN-NaCN mixtures.

[0057]FIG. 2 illustrates silver elution from anion exchange resins using CuCN-NaCN mixtures.

[0058]FIGS. 3A and 3B respectively illustrate gold and silver elution incorporating continuous cementation using copper powder.

[0059]FIG. 4 illustrates copper elution using H₂SO₄—H₂O₂ mixtures.

[0060]FIG. 5 provides a flow-chart illustrating the recovery of gold and silver using resin technology.

BEST MODE OR MODES OF PERFORMING THE INVENTION

[0061] The present invention is concerned with procedures designed to elute precious metals, notably gold, silver and platinum group metals, from resins at any stage in the adsorption cycle, but particularly from resins that have reached a gold level of the order of 10 kg/tonne of dry resin. At this point the resins have possibly been subjected to several base metal elutions by procedures outlined in accordance with the second embodiment of the invention with regard to decoppering, because it would be usual to have conducted such procedures where the copper or other base metal concentrations were significantly higher than gold for example.

[0062] The present invention details a new procedure for eluting gold and other precious metals from such loaded resins that departs significantly from current practice. It will be appreciated that the current precious metal elution systems for strong base anion exchange resins rely on the ability of strong alkaline solutions containing the zinc tetrahedral tetracyanide anion to replace the linear precious metal cyanoanions from the resin. In one form of the invention it is proposed to use a typically linear singly charged anion to undertake the same task and at the same time remove the problem of having to cope with the very high introduced concentrations of the CN anion and the disposal of the remnant zinc cations so introduced to the system.

[0063] The present invention comprises a series of operations whereby the adsorbed precious metals, typically gold, silver and platinum group metals, are selectively removed from the loaded medium in a manner which delivers a strong aqueous solution of those metals in a form capable of being further treated by known electro or hydrometallurgical procedures for their recovery in metallic form. The eluant consists of an aqueous solution containing a complex of formula MX₂—, wherein M is a metal of group IB or IIB of the Periodic Table of Elements, and X is a monovalent radical.

[0064] The medium may be an ion exchange medium or activated carbon, wherein if an ion exchange medium, it may be of a salt, acid or base in nature, and is generally solid in form. Preferably, the ion exchange medium is an anion exchange medium, and is usually in the form of an anion exchange resin. In its most common format the anion exchanger (the functional group) is affixed to the surface of an insoluble base support, frequently presented in the form of porous beads of cross linked polystyrene which leads directly to the commonly used term “anion exchange resin”.

[0065] Strong base anion exchange resins are mostly based on porous styrene-DVB copolymers to which a charged functional group has been chemically attached, and for a typical Type I resin this group is tri-methyl ammonium chloride, N⁺(CH₃)₃Cl⁻. The positively charged tri-methyl ammonium entities are firmly bonded to the resin matrix, and the negative chloride anions are held by strong electrostatic forces within a thin diffuse aqueous layer, within which they, and other anions, are free to migrate under the restrictions imposed by the requirement to maintain a condition of electoneutrality.

[0066] The anion exchanger functional group of an ion exchange resin for use in the process of the present invention may be a strongly basic benzyltrimethylammonium chloride. In using benzyltrimethylammonium chloride, the Cl⁻ counter ion is exchanged with the various base and precious metal cyano complex anions. This particular functional group is chemically one of the most stable of the many possible amine substitutional products in this group, and provides a highest degree of dissociation for the counter anions over a wide range of operating pH.

[0067] In the operation of the process of the present invention, resin will be loaded with base and precious metals from a pregnant solution being passed over a resin bed in either a column contactor or stirred reactor mode. Preferably, a pregnant solution or slurry derived from the cyanide leaching of a suitable ore is passed through a resin filled column, or a cascade arrangement of stirred liquid/solid reactors resulting in the base metal and precious metal components being adsorbed onto a commercially available resin source, such as Dowex MSA1 (Dow Chemical Co., Midland, Mich. USA).

[0068] When exposed to pregnant aqueous solutions bearing dissolved precious metals, these strong base anion exchange resins uptake the complex ions which displace the chloride or any other simple anion such as hydroxide, bisulphate or sulphate, which may be uptaken on the resin.

[0069] After adsorbing the required mass of precious metals, the mass is referred to as being “loaded”. The mass of loaded resin contained within the column or stirred reactor is then isolated from the pregnant liquor stream for subsequent washing and elution in an aqueous eluant stream containing the complex anion of formula MX₂—.

[0070] The complex anion of formula MX₂—, is generally linear in nature, wherein the metal M is selected from the group consisting of: copper(I), gold(I), silver(I), cadmium(I) and mercury(I), and X corresponds to a monovalent anion selected from the group consisting of: halides, cyanide, thiocyanate and isothiocyanate. The halide may be selected from the group consisting of: chloride, bromide, iodide and fluoride, and is generally cyanide or thiocyanate. Preferably, the complex anion of formula MX₂— is either Cu(CNS)₂— or Cu(CN)₂—, but Cu(CN)₂— is generally preferred.

[0071] Further, the aqueous solution of the complex anion of formula MX₂— also includes a counterion. The counterion may be any counterion which does not cause precipitation of the complex anion of formula MX₂—. For example, such a counterion may include an alkali metal cation, for instance, Na⁺or K⁺, or an ammonium ion.

[0072] Preferably, the aqueous solution containing a complex anion of formula MX₂— also contains a salt of formula M′X, wherein said M′ is the counter cation in relation to the complex anion of formula MX₂—. The amount of the salt of formula M′X may range from between about 0 to about 5 moles per mole of the complex anion of formula MX₂—, but is generally about 1 mole per mole of the complex anion of formula MX₂—.

[0073] The minimum amount of aqueous solution containing a complex anion of formula MX₂— used in the present invention is that sufficient to elute the precious metals from the ion exchange medium or activated carbon medium. The upper limit of the concentration of the complex anion of formula MX₂—in the aqueous solution is that wherein the complex anion of formula MX₂— remains soluble. Preferably, the concentration of the complex anion of formula MX₂— in the aqueous solution is at least about 0.01M, but may range between about 0.01M to about 2M, to between about 0.1M to about 0.5M.

[0074] Usually, the aqueous solution of the complex anion of formula MX₂— is provided by preparing an aqueous solution of a salt M′X at the desired concentration, wherein M′ is the desired counterion, and adding a salt MX in a sufficient quantity to provide the desired concentration of M, and hence the complex anion of formula MX₂—, in solution.

[0075] Alternatively, an aqueous solution of MX may be prepared first, with subsequent addition of M′X, or both MX and M′X may be added to water together, or any combination of these steps may be used.

[0076] A typical eluant is made from 0.5M CuCN and 1.0M NaCN, and may be passed through the column at for example, 4 bed volume/h, for a time period depending on the amount of gold loading, and the elution rate (for example 48 hours). The elution process may proceed as a series of elution cycles, or a continuous passage of recycled eluant solution.

[0077] In terms of a typical eluant, 0.5M CuCN-1.0M NaCN for example, contains a very high proportion of Cu(CN)₂— species. This species is single-charged and linear in structure, and accordingly, easily displaces other single-charged complex ions, such as, Au(CN)₂— and Ag(CN)₂—. Effectively, during the elution process, the precious metal cyanide ions are displaced by copper cyanide ions.

[0078] Other reaction conditions for carrying out the process of the present invention are not critical. The process may be carried out at ambient temperature, or elevated temperatures, but is typically carried out at ambient temperature. The pH is usually the pH of the aqueous solution of the complex anion of formula MX₂— as prepared, without any further pH adjustment.

[0079] The results of typical gold and silver elutions are illustrated in FIGS. 1 and 2.

[0080] The process of the present invention is useful for mediums loaded with precious metal-cyano complexes via both clear and slurried solutions.

[0081] Eluted liquor may then be passed onto a cementation cell. Cementation is a redox process in which a metal, such as zinc and/or copper, is used to remove precious metal ions, such as gold, silver and platinum groups metals, from solution. In terms of a cyanide system, the recovery of gold can be illustrated by the following equation:

Cu+Au(CN)₂—→Au+Cu(CN)_(2—)

[0082] In such a cementation cell, copper may be added initially, with further additions after a series of time intervals. Both the cementation cell and resin column may be operated in series.

[0083] Once the precious metals have been eluted off the medium, they may then be cemented onto copper metal by methods generally known in the art. Precious metal/copper cement can then be easily purified by dissolution of copper in a mixture of sulphuric acid and an oxidant (such as hydrogen peroxide or air), leaving behind a product rich in precious metals for smelting into dore. Further, methods for dissolution of copper from cement containing precious metals are also generally known in the art.

[0084] The results of a typical combined gold and silver elution and copper cementation are illustrated in FIGS. 3A and 3B. respectively. The process engineering conditions chosen for contacting the resin and solution will be a design variable but, as described above, preferably will incorporate either a column resin packed bed or some suitable form of stirred liquid/solids reactor configured for either batch or continuous operation.

[0085] In the manner outlined above, in an example of the operation of the process of the present invention, resin will be loaded with base and precious metals from a pregnant solution being passed over a resin bed in either a column contactor or stirred reactor mode. When exposed to pregnant aqueous solutions bearing dissolved copper, precious metal or other base metal cyano complexes, these strong base anion exchange resins adsorb the complex ions which displace the chloride or any other simple anion such as hydroxide, bisulphate or sulphate which may be adsorbed on the resin.

[0086] Preferably, and in accordance with the relevant ratio of copper and other base metal cyanides to the precious metal cyanides, the resin capacity for the base metals will be reached and the flow will have to be terminated. The loaded resin charge is then taken off line and subjected to a process to remove the base metal cyanides from the resin.

[0087] Typically, the resin is washed with water and then subjected to a sequence of elution cycles or a continuous passage of a recycled eluant solution depending on the objectives at the time. The eluant may comprise an aqueous solution of 5% by weight sulphuric acid, with an addition of hydrogen peroxide as oxidant to a level which is consistent with the requirement to preserve the structural and chemical integrity of the anion exchange resin and the functional group attached. The optimum peroxide addition for each variety of commercial resin has to be determined for the specific conditions, but typically a range of between 0.5 to 1.0% by weight is utilised with the Dowex commercial resin (Dowex MSA1-Dow Chemical Co., Midland, Mich. USA).

[0088] The eluate produced from this operation is an acidic aqueous copper sulphate with relevant other base metal sulphates which, utilising known recovery procedures, may be treated to provide copper metal and return the acid to the elution circuit. The acid serves to regenerate the stripped resin ready for its return to adsorption from the pregnant liquor streams. The results of a typical copper elution are presented in FIG. 4 where it is shown that the adsorbed precious metals, gold and silver, remain on the resin largely untouched during the base metal elution cycle.

[0089] For example, an anion exchange resin loaded with copper, gold and silver cyano complexes is exposed in an appropriate manner to an aqueous solution containing hydrogen peroxide and sulphuric acid for a period of time sufficient to provide for the desired removal of a predetermined quantity of the adsorbed copper complex. The process engineering conditions chosen for contacting the resin and solution will be a design variable but preferably will incorporate either a column resin packed bed or some suitable form of stirred liquid/solids reactor configured for either batch or continuous operation. The respective concentrations and molar ratios of the hydrogen peroxide and sulphuric acid required for a successful chemical conversion of the adsorbed copper species are limited at the upper scale by the need to control the oxidation potential of the solution to reflect the specific chemical stability of the ion exchanger functional group and its polymer base support. A preferred feature for the preparation of a satisfactory aqueous oxidising solution is a solution which will perform the Cu(I) to Cu(II) oxidation without in so doing irretrievably destroy either the anion exchange capacity of the resin functional group or the physical and mechanical properties of the resin.

[0090] In this instance, copper is present in the lower oxidation state, as Cu(I), in the copper cyano complexes and the process provides for the direct chemical oxidation of the metal to the Cu(II) state and the destruction of the adsorbed copper cyano complexes attached to the resin active sites, and permitting a level of copper removal which approaches complete. The copper is then free to enter the aqueous phase as the cation Cu²⁺ and accordingly is not available for capture by the strong base anion exchanger. The recovery of copper metal from the resultant acid solution is then achieved by the application of known technology in this area, notably electrowinning to cathodic copper or cementation onto scrap ferrous metal or precipitation as copper hydroxide according to the scale and economics of the operation.

[0091] This decoppering stage of the present invention further provides for the control of the conditions for the recovery of the cyanide component of the copper cyano complexes either as a solution containing the less toxic cyanate anion CNO— or as the gas HCN.

[0092] Experimentation with different commercially available strong base resins has revealed some degree of variation between resin types. As an example one common such resin released 68% by weight of its loaded copper into the Cu²⁺ state after one hour of reaction in a solution containing 0.5% by weight hydrogen peroxide and 5% by weight sulphuric acid. The washed resin when reloaded from the original pregnant solution loaded copper in a normal manner, which is an indication that the resin is capable of withstanding degradation in this strength of oxidising solution. The acidic liquor from this procedure was suitable for direct recovery of the copper values therein by any of the metallurgical procedures mentioned previously.

[0093] Further, the copper loaded resin may be contacted in the first instance with a mineral acid, preferably sulphuric acid at an aqueous concentration sufficient to provide a pH of less than between 6, whereby the copper cyano complex adsorbed onto the resin will have been converted to the CuCN and/or Cu(CN)₂— state. During this step a proportion of the cyanide will have been liberated as HCN from the adsorbed copper cyano complex and this product is preferably removed from the system along with the acid solution for recovery of its cyanide by either passing directly into a solution of calcium hydroxide or optionally into an air purged volatilisation column where the HCN gas is removed into a caustic soda scrubbing tower for the regeneration of sodium cyanide. Subsequent oxidation of the adsorbed copper cyano complex or CuCN left on the resin is performed as outlined in accordance with the second or third embodiments of the invention, in an oxidising solution which is optionally constituted in a different manner to take advantage of the higher copper to cyanide ratio in the adsorbed copper cyano complex or CuCN.

[0094] Optional features of this process provide for the use of oxidant solutions derived from Caro's acid (H₂SO₅), acid ferric sulphate and other solutions capable of converting the copper in the cyano complex from its Cu(I) univalent state to the cationic Cu(II) state without adversely affecting the physical and chemical properties of either the anion exchange functional group or its polymer base support.

[0095] Alternative arrangements are also provided for the application of this process to the recovery of copper and cyanide from copper contaminated water wherein the copper is present as a soluble cyano complex. In such instances the further recovery of any precious metals similarly complexed with cyanide is also envisaged.

[0096] In summary, FIG. 5 provides a flow-chart describing typical major unit operations (adsorption, precious metal elution, copper elution and gold cementation) in the recovery of precious metals and copper from the medium, in this case, anion exchange resin.

[0097] Some of the advantages of the present invention include the following:—

[0098] 1. Simplicity, wherein only dilute cyanide streams are to be handled, that is, HCN is generated mainly from base metal cyanides (Cu, Zn, etc.) and not from strong cyanide eluant solutions;

[0099] 2. The precious metal containing eluant is suitable for a precious metal recovery stage to be conducted by simple cementation onto copper.

[0100] The invention will now be described in greater detail by reference to specific examples, which should not be construed as limiting on the scope thereof.

EXAMPLES Example 1 Loading of Resins

[0101] Aim

[0102] To load resins with Au, Ag, Cu and Zn, from a plant liquor, and determine loading capacity of Dowex MSA1 (Dow Chemical Co., Midland, Mich. USA) resin.

[0103] Procedures

[0104] Twenty litres of plant liquor (May Day Mine) were used for the loading test. Since the level of gold in the original liquor is low (about 1.8 mg/L Au) an aliquot of 1,100 mg/L Au in 0.O1M KCN was added to increase the concentration of gold tenfold.

[0105] 100 g of dry resin (a macroporous strong-base resin from Dow Chemical Co., Midland, Mich. USA)(Dowex MSA1) was added to the stirred solution (5g/L dry resin loading) and left for 24 hours.

[0106] The final solution was analysed for Au, Ag, Cu, Zn and free cyanide.

[0107] Results

[0108] The loadings of metal ions after 24 hours are: 3.59 kg/t Au, 10.84 kg/t Ag, 34.2 kg/t Cu and 8.02 kg/t Zn from the original solution of 18.6 mg/L Au, 57.75 mg/L Ag, 172.6 mg/L Cu and 40.3 mg/L Zn.

[0109] The residual concentrations of the final solution are: 0.642 mg/L Au, 3.56 mg/L Ag, 1.61 mg/L Cu and 0.22 mg/L Zn, indicating conditions close to maximum loading capacity of the resin.

[0110] Analysis of free cyanide shows concentration dropping from 318 mg/L to 138 mg/L CN (loading to 35.9 kg/t CN).

[0111] Loading capacity is calculated for each adsorbed species. Single-charged ions has no. of milli-equivalent equal to no. of milli-mole whereas double-charged ions (Zn) have m.eq equal to 2xmilli-mole.

[0112] The total metal loading capacity is 0.900 m.eq/g resin. Taking into account the loading of free cyanide (1.38 m.eq/g resin) the total operating capacity is 2.38 m.eq/g, compared to the max. loading capacity quoted by the manufacturer (Dow Chemical Co., Midland, Mich. USA) as 4.0 m.eq/g resin.

[0113] The results are summarised in the Table 1 below: TABLE 1 Results of Loading Test Au Ag Cu Zn Free CN Recovery % 96.6 93.8 99.1 99.5 56.4 Loading (kg/t) 3.59 10.84 34.2 8.02 35.9 Loading capacity 0.0182 0.100 0.538 0.244 1.38 (meq/g resin)

Example 2

[0114] Aim: To elute gold and silver cyanide from loaded resins prepared using CuCN-NaCN mixtures.

[0115] Bottle roll tests were used for this experiment. Volume of eluant: 200 mL, Resin loading: 10 g dry resin/L eluant, Resin used: Dowex MSA1.

[0116] Samples (4 mL each) taken at time intervals were analysed by AAS for Au, Ag, Zn and Cu. Free cyanide was determined by potentiometric titration.

[0117] Four elution systems were tested: System 1: 0.5M CuCN - 1.0M NaCN System 2: 0.5M CuCN - 2.0M NaCN System 3: 0.25M CuCN - 0.50M NaCN System 4: 0.1M CuCN - 0.2M NaCN

[0118] Results

[0119] Au and Ag Elution

[0120] As outlined in FIGS. 1 and 2, and Tables 2 and 3 below, both System 1 (0.5M CuCN-11.0M NaCN) and System 3 (0.25M CuCN-0.5M NaCN) achieved around 76% elution in 6 hours. A lower equilibrium elution was reached for System 4 (0. 1M CuCN-0.2M NaCN). In excess NaCN, lower elution efficiency and equilibrium were achieved (System 2).

[0121] At maximum elution, the amount of gold eluted was 76% of the Au(I) loading capacity of 0.0182 m.eq/g resin, equal to 0.0138 m.eq/g resin. The high concentrations of gold in the final eluant (>20 mg/L Au(I)) probably prevented further gold being eluted.

[0122] The same trend (lower elution with excess NaCN) was observed for Ag elution with >95% Ag eluted in 2 hours for System 1 vs 27% Ag elution in System 2. A slightly slower elution rate was observed with System 3 (95% Ag eluted after 4 hours). System 4 yielded 78% after 6 hours.

[0123] The amount of Ag eluted in best conditions (99%) was equivalent to 0.099 m.eq/g resin.

[0124] Table 2 summarises the metal ion loadings of the original resin and the elution efficiency after 6 hours for comparison of the 4 eluant systems tested. TABLE 2 Original Conditions and Elution Efficiency of Various Systems after 6 hours CONDITIONS Au(I) Ag(I) Zn(I) Original Resin Loading (kg/t) 3.59 10.84 8.02 Elution efficiency (%) after 6 hours for: 0.5M CuCN - 1.0M NaCN 76.0 98.0 3.8 0.5M CuCN - 2.0M NaCN 9.50 34.1 6.9 0.25M CuCN - 0.50M NaCN 76.3 96.2 0.1 0.1M CuCN - 0.2M NaCN 41.7 78.9 1.8

[0125] The results of all ion exchange actions after 6 hours are also summarised in Table 3 below. The convention for signs are (+) for loading and (−) for elution. TABLE 3 Amounts Eluted (−) or Loaded (+) in kg/t (or m.eq/g resin in brackets) System CuCN NaCN Au(I) Ag(I) Zn(II) CN Cu(I) 1 0.5M 1.0M −2.73 −10.62 −0.305 n/a n/a 2 0.5M 2.0M −0.341 −3.69 −0.553 n/a n/a 3 0.25M  0.50M  −2.73 −10.43 −0.008 −38.5 +177 (−0.0138) (−0.096) (−0.0002) (−1.48) (+2.78) 4 0.1M 0.2M −1.49 −8.55 −0.145 +72.6 +27.0 (−0.0076) (−0.0793) (−0.0044) (+2.79) (+9.418)

[0126] From examination of these results, a very favourable system is System 3 (0.25M CuCN+0.50 M NaCN). In System 3, more gold and silver were eluted with Cu(I) cyanide without being taken-up by the resins.

Example 3 Testing of Various Eluants

[0127] Aim:

[0128] To test the elution characteristics of various acid mixtures which are known to be strong oxidants.

[0129] Resin having head grade: 32.2 kg/t Cu and 3.46 kg/t Au

[0130] Procedures:

[0131] Soak resins (0.266-0.311 g) in various solutions as specified in Table 4 below for one hour. Conditions were not optimised. Samples were taken after 1 hour for analysis using atomic absorption spectrophotometer for dissolved Cu and Au analysis. The total amounts of Cu and Au in solution were used to calculate the percentage of elution (recovery) as shown in Table 4.

[0132] Results: TABLE 4 Elution of copper and gold by various acid mixtures Cu Au Resin wt Cu Au Elution Elution Eluant and volumed used g mg mg % % 1. HNO₃ (3 ml) + 0.311 10.043 0.057 100 5.3  HCl (6 ml) 2. HCl (10 ml) + 0.337 10.934 0.009 100 0.7  H₂O₂ (5 ml) 3. H₂SO₄ (8 ml) + 0.324 10.142 0.100 97.2 8.9  H₂O₂ (4 ml) 4. HNO₃ (10 mL) 0.266  8.250 0.361 96.4 39.3

[0133] HNO₃: Nitric acid, HCl; Hydrochloric acid, H₂SO₄: sulphuric acid

[0134] Comments:

[0135] Mixture 1 is traditionally known as aqua regia, a powerful oxidising solution. Solution 4 is concentrated nitric acid, also a strong oxidising acid. Mixtures 2 and 3 were made with hydrochloric acid and sulphuric acid, respectively with hydrogen peroxide, H₂O₂ as oxidising agent.

[0136] In Mixture 2, hydrogen peroxide is also capable of oxidising Cl ions to chlorine, and therefore is not a good mixture to be used. For Mixture 3, sulphate ions are inert.

[0137] Conclusions:

[0138] In all cases, copper was almost fully eluted and strong oxidising solutions will also elute gold from the resins, except Mixture 2 where most hydrogen peroxide reacted with chloride ions, reducing its impact on gold elution.

Example 4 Elution-Cementation Circuit for Gold and Silver Recovery—Column Arrangement

[0139] A 150 ml column was used for this test. The column was filled with 106 g wet (70 g dry) commercial resin (Dowex) (Dow Chemical Co., Midland, Mich. USA). The eluant which was made from 0.5M CuCN and 1.0M NaCN was passed through the column at 4 bed volume/h (or 0.755 L/h) for a period of 48 hours. The eluted liquor was passed onto a cementation cell having retention volume of 342 mL (27 min). In this cell, 10 g of copper was added initially, with further additions of 10 g after 6 hours and 5 g after 24 hours. Both the cementation cell and resin column were operated in series.

[0140] Samples taken from the liquor stream leaving the column and the cementation cell at different intervals were then analysed for Au and Ag. The stripped resin was also assayed for Cu, Zn, Ag and Au (2 g of dried resin was digested in 20 mL of aqua regia for 30 min on a hot plate. Solutions were then cooled and diluted before analysis by atomic adsorption spectroscopy). The copper cement containing gold was digested in different acids (nitric acid and/or hydrogen peroxide/sulphuric acid to dissolve copper and silver and aqua regia to dissolve gold) and analysed to determine the masses of gold and silver in the cement for mass balance checking.

[0141] Metal loading of resin used 13.1 kg/t Au, 19.9 kg/t Ag, 12.1 kg/t Zn and 57.9 kg/t Cu (Dowex MSA1 resin)

[0142] Volume of eluant used: 1 litre (recycled)

[0143] Results:

[0144] 1. Elution Kinetic Data (Concentration of Au and Ag Only vs Time): Showing Slower Kinetics of Elution for Gold. TABLE 5 Elution Kinetic Data Au leaving Au leaving Ag leaving Ag leaving resin column cementation resin column cementation Time (h) (ppm) cell (ppm) (ppm) cell (ppm) 0 0 <0.02 0 0 0.5 105 <0.02 1318 1.82 1.0 166 <0.02 964 1.54 2 159 1.0 681 0.52 4 124 7.4 94 0.01 6 76 14.3 7.8 <0.01 8 55 0.98 5.0 <0.01 24 17.0 3.45 1.98 <0.01 36 12.5 2.53 <0.01 <0.01 48 3.1 0.33 <0.01 <0.01

[0145] 2. Assays of the Eluted Resin

[0146] Analysis of the eluted resin yielded the residual metal loading of the stripped resin. Results showed complete elution of Au and Ag, whereas Zn was not eluted. TABLE 6 Elution Efficiency Metal Residual loading, kg/t Elution efficiency, % Au 0.27 97.9 Ag 0.2 98.9 Zn 11.9 1.6 Cu 167.9

[0147] 3. Analysis of Au and Ag in Copper Cement Yielded 878 mg Au and 1,384 mg Ag, Confirming Elution Efficiencies of 96.1% for Au and 99.5% for Ag.

Example 5 Elution of Copper Using Various Mixtures of Different Acids and Hydrogen Peroxide

[0148] Aim:

[0149] To test the elution characteristics of mixtures of sulphuric acid (10 or 50 g/L) and hydrogen peroxide (0.15 or 0.75 g/L) and nitric acid (10 or 50 g/L) and hydrogen peroxide.

[0150] Procedures:

[0151] Samples of resins (about 0.2 g) were added to 100 mL of eluants in plastic bottles and rolled for 24 hours. Samples were taken after 210 minutes and at 24 hours for analysis of Cu and Au using AAS.

[0152] Resin head grade: 32.2 kg/t Cu and 3.46 kg/t Au.

[0153] Results:

[0154] Results of the testwork after 210 minutes are summarised in Table 7 below. Gold was eluted from the resins if nitric was used (up to 37.8% in 50 g/L nitric acid). In the case when sulphuric acid was used, minimum gold (<1.5%) was eluted after 210 minutes. TABLE 7 Elution results after 210 minutes Resin Cu Au Cu Au wt eluted eluted elution elution Eluant g mg mg % % 0.15 g/L H₂O₂ + 10 g/L 0.2102 5.36 0.005 79.1 0.7 sulphuric acid 0.75 g/L H₂O₂ + 50 g/L 0.2296 7.14 0.012 96.6 1.5 sulphuric acid 0.15 g/L H₂O₂ + 10 g/L 0.2203 6.19 0.065 96.2 9.4 nitric acid 0.75 g/L H₂O₂ + 50 g/L 0.2137 6.83 0.279 99.3 37.8 nitric acid

[0155] After 24 hours, 100% copper was eluted. Elution of gold remained the same: 0.5%, 1.2%, 9.7% and 36.8%, respectively for the four eluants above.

Example 6 Elution of Copper Using an H₂SO₄-H₂O₂ System (Column Test)

[0156] Aim:

[0157] To test the elution of a commercial Dowex MSA-1 resin loaded to the extent of 52 kg/t Cu, 6.16 kg/t Ag and 0.60 kg/t Au.

[0158] Procedure:

[0159] A 70 gram (dry) sample of the loaded resin was eluted in a 150 mL column using 2 litres of aqueous solution, passing upwards through the column in a single pass mode at a rate of 10 bed volumes per hour (1.404L/hr).

[0160] Results

[0161] The progress of the removal of the adsorbed metals is illustrated in FIG. 4. It was observed that the major proportion of adsorbed copper was removed within one hour, and after two hours the efficiency had reached 100%.

[0162] Gold and silver however, were left largely untouched on the resin as evidenced in FIG. 4.

INDUSTRIAL APPLICABILITY

[0163] The present invention relates to a hydrometallurgical process for selectively recovering precious metals from a medium loaded with precious metals and base metals. Further, the present invention also relates to a process for eluting copper from a medium loaded with precious metals, copper and optionally, other base metals. 

1. A process for the selective recovery of precious metals from a medium loaded with precious metals and base metals, wherein said selective recovery is achieved by exposing said medium to an aqueous solution containing a complex of formula MX₂—, wherein M is a metal of group IB or IIB of the Periodic Table of Elements, and X is a monovalent radical.
 2. The process of claim 1, wherein said precious metal is selected from the group consisting of: gold, silver and platinum metals group.
 3. The process of claim 1 or 2, wherein said medium is an ion exchange medium or activated carbon.
 4. The process of claim 3, wherein said ion exchange medium is an anion exchange medium.
 5. The process of claim 4, wherein said anion exchange medium is in the form of an anion exchange resin.
 6. The process of any one of claims 3-5, wherein said ion exchange medium has an anion exchange functional group which is benzyltrimethylammonium chloride.
 7. The process of any one of claims 1-6, wherein said metal M of formula MX₂— is selected from the group consisting of: copper(I), gold(I), silver(I), cadmium(I) and mercury(I).
 8. The process of any one of claims 1-7, wherein said X of formula MX₂— corresponds to a monovalent anion selected from the group consisting of: halides, cyanide, thiocyanate and isothiocyanate.
 9. The process of claim 8, wherein said halide is selected from the group consisting of: chloride, bromide, iodide, fluoride, cyanide and thiocyanate.
 10. The process of any one of claims 1-9, wherein said complex anion of formula MX₂— is either Cu(CNS)₂— or Cu(CN)₂—. 11 The process of claim 10, wherein said complex anion of formula MX₂— is Cu(CN)₂—.
 12. The process of any one of claims 1-11, wherein said complex anion of formula MX₂— is linear.
 13. The process of any one of claims 1-12, wherein said aqueous solution of said complex anion of formula MX₂— also includes a salt of formula M′X, wherein said M′ is the counter cation in relation to the complex anion of formula MX₂—.
 14. The process of any one of claims 1-13, wherein the concentration of said complex anion of formula MX₂— in said aqueous solution is that sufficient to elute said precious metals from said medium.
 15. The process of any one of claims 1-14, wherein the concentration of said complex anion of formula MX₂— in said aqueous solution is not more than that wherein said complex anion of formula MX₂— remains soluble.
 16. The process of claim 14 or 15, wherein said concentration of the complex anion of formula MX₂— in the aqueous solution is at least about 0.01M.
 17. The process of any one of claims 14-16, wherein said concentration of the complex anion of formula MX₂— in the aqueous solution is in the range of between about 0.1M to about 0.5M.
 18. A process for the elution of copper from a medium loaded with precious metals, copper and optionally other base metals, wherein said elution is achieved by exposing said medium to an aqueous solution capable of oxidising Cu(I) to Cu(II), whilst not eluting said precious metals from said medium.
 19. A process for the elution of copper from a medium loaded with precious metals, copper and optionally other base metals, wherein said process comprises: (a) selectively recovering said precious metals from said medium in accordance with the process of any one of claims 1-17, and (b) eluting said copper from said medium by exposing said medium to an aqueous solution capable of oxidising Cu(I) to Cu(II).
 20. The process of claim 18 or 19, wherein said aqueous solution capable of oxidising Cu(I) to Cu(II) comprises an oxidising agent, together with an acid.
 21. The process of claim 20, wherein said oxidising agent is selected from the group consisting of: hydrogen peroxide, Caro's acid (H₂SO₅) and acid ferric sulfate.
 22. The process of claim 20 or 21, wherein said aqueous solution capable of oxidising Cu(I) to Cu(II) comprises an amount of hydrogen peroxide between about 0.25-2% by weight hydrogen peroxide and about 1-10% by weight acid.
 23. The process of claim 20 or 21, wherein said aqueous solution capable of oxidising Cu(I) to Cu(II) comprises about 0.5% by weight hydrogen peroxide and about 5% by weight acid.
 24. The process of any one of claims 20-23, wherein said acid is a mineral acid.
 25. The process of claim 24, wherein said acid is sulfuric acid.
 26. The process of any one of claims 19 to 25, wherein prior to exposing said medium to said aqueous solution capable of oxidising Cu(I) to Cu(II), said medium is contacted with a mineral acid.
 27. A process for the selective recovery of precious metals from a medium loaded with precious metals and base metals comprising: (b) eluting copper from said medium in accordance with the process of claim 18, wherein said elution of copper occurs prior to said selective recovery of precious metals, and subsequently (b) selectively recovering precious metals by exposing said medium to an aqueous solution containing a complex of formula MX₂—, wherein M is a metal of group IB or IIB of the Periodic Table of Elements, and X is a monovalent radical, in accordance with the process of any one of claims 1 to
 17. 